Current understanding of physical and biogeochemical processes from coastal to deep-sea environment is limited. This is mainly due to the constraints in current sensing techniques. The major goal of ocean research is to develop systems that allow measurement of significant and complementary ocean parameters throughout large volumes and over large time spans. To this end researchers have made significant investments in remote sensing, AUVs, ROVs and buoys. These technologies sense local environmental variables at a singe point in space-time. However, the use of multiple vehicles improves the measurement quality. But, the gain from higher spatial sampling frequency is directly related to the number of additional vessels used. Using more support vessels, whether AUVs or ships, will add to the cost. While remote sensing and in situ buoy systems have provided part of the solution, both have limitations in terms of energy consumption and non-steady responses.
In addition to monitoring the important ocean processes, pressure/depth sensors also find use in maritime/homeland port security. Detecting the movement of surface vessels such as ships, boats, or buoys and subsurface systems such as submarines, unmanned submerged vehicles, mining systems or surveillance systems have become important in current times. Conventional techniques involve either an acoustic or nonacoustic method. Even though these are highly effective in single point/sector detection in time-space, their ability for wide area surveillance/dynamic monitoring is extremely limited. There are several emerging techniques for detecting submarines ranging from direct detection of submarine structure to indirect techniques through analysis of the effect the submarine has on the surrounding ocean environment (wave or thermal variations). One such indirect method involves measurement of environmental variations due to the submerged vehicle such as water/air temperature changes versus depth, pressure variations and wave height changes. The physical surface characteristics range from a wake developed by the moving vessel, detectable on the surface to generation of internal waves, which are manifested through subtle surface effects. However, these effects are highly variable, dependent on the vessel's operational parameters such as speed, size and depth. Of all the physical effects, detection of internal waves is the most realistic technique for a wide area, as other effects are severely constrained by vessel speed and depth. Internal waves are periodic disturbances in temperature and density (or pressure, P=ρgD, where ρ is the density of water and P is the pressure) of water at depths, where temperature drops and density rises sharply with increasing depth. In short, measuring and mapping (1) pressure and wave variations in sea surface/sub-surface and (2) temperature changes in water and atmosphere will be an indispensable surveillance tool to detect the movement of both surface and submerged vessels. Another application for pressure/depth sensors is tsunami wave detection at thousands of meters in water, with very high resolution (<0.001% FS), accuracy, stability, and insensitivity to environmental disturbances. Commercial single diaphragm piezoresistive pressure/depth sensors are capable of measuring up to 10,000 bar with an accuracy of 0.015% of full scale.
Accordingly, what is needed in the art is an improved piezoresistive pressure sensor for dynamic, dense and distributed ocean sensing applications having improved sensitivity and wider full scale span compared to the conventional single diaphragm piezoresistive pressure sensors currently known in the art.